1. Field of the Invention
The present invention generally relates to the detection of defects and the prediction of electromigration behavior of metal interconnects using an electrical noise technique, and, more particularly, to a dual channel D.C. noise system in which the system-induced noise level at a wide range of current and temperature conditions is significantly less than Johnson noise at all frequencies of interest.
2. Description of the Prior Art
As the cross-section of metal interconnects in integrated circuits continues to decrease and the resulting current density increases, failure of the metallization due to electromigration remains a concern. The susceptibility of metal interconnects to electromigration failure is conventionally evaluated by subjecting many samples to conditions of accelerated current and temperature until failure results in the form of an open or short (extrusion), e.g. a Median Time to Fail (MTTF) test. These results are then extrapolated to use-conditions. In addition to being destructive, this test can take weeks or even months to complete. This implies that if a problem with the metallization is uncovered with a MTTF test, there may be a large number of parts already manufactured that may be considered unsuitable for use.
Another factor that can limit the lifetime of metal interconnects is the presence of defects that may result from the manufacturing process. These defects (e.g. notches and scratches), reduce the cross-sectional area of the interconnect with a concomitant increase in current density, which may result in an early electromigration failure. Conventional measurements such as resistance are generally unable to detect the presence of these defects since, while a significant (e.g. likely to compromise performance of the circuit over a long period of service) notch or scratch may reduce the cross-sectional area of a conductor by 50% to 90%, the dimension of the notch or scratch along the length of the conductor is typically very short (e.g. much less than 1% of the length of the conductor).
It is therefore desirable to develop a technique that can evaluate the reliability of metal interconnects quickly and non-destructively. One technique that has received a great deal of attention recently is 1/f noise or, more generally, excess noise which characteristically varies as 1/f.sup..alpha., where .alpha. usually lies in the range 0.7.ltoreq..alpha..ltoreq.1.4. Published reports have shown a correlation between interconnect reliability and excess noise [see for example, J. L. Vossen, Applied Physics Letters, Vol. 23, p. 287 (1973), M. I. Sun, et al., 10th International Conference on Noise in Physical Systems, Budapest, Hungary, p. 519 (1989), Z. Celik-Butler, et al., Solid-State Electronics, Vol. 34, No. 2, p. 185 (1991), M. L. Dreyer, MRS Symposium Proc., Vol. 225, p. 59 (1991)].
One problem with most noise measurement systems is that the background, or system-induced, noise exceeds the Johnson noise, sometimes referred to as thermal noise, of the device under test. This background noise prohibits the accurate measurement of samples with excess noise close to Johnson noise. The magnitude of this background noise is generally observed to increase as the frequency decreases in the same manner as the excess noise of interest, whereas the Johnson noise is independent of frequency [see for example, A. Diligenti, et al., IEEE Electron Device Letters, Vol. EDL-6, No. 11, p. 2487 (1985), T. M. Chen, et al., IEEE IRPS, p. 87 (1985), G. M. Gutt, et al. 1EEE Southeastcon Proc., p. 1305 (1989), D. M. Liou, et al., Japanese Journal of Applied Physics, Vol. 29, No. 7, p. 1283 (1990), W. Yang, et al., IEDM Proceedings, p. 681 (1989), J. Komori, et al., IEEE International Conference on Microelectronic Test Structures, Vol. 4, No. 1, p. 257 (1991), J. A. Schwarz,.et al., J. Appl. Phys., Vol. 70, No. 3, p. 1561 (1991)]. In other words, if the noise of the measurement system is too high it will mask the excess noise (above the Johnson noise) of the device under test. Minimizing the background noise of the measurement system is especially important for the study of excess noise in metal interconnects since the excess noise magnitude tends to be only a factor of 10 to 100 above the Johnson noise at 1 Hz.